members of the class Cyclostomata, such as Petromyzon (lamprey), are jawless vertebrates that are primarily marine but migrate to freshwater for spawning . After spawning, the adults die within a few days, and their larvae—specifically the ammocoetes larvae in the case of Petromyzon—undergo metamorphosis before returning to the ocean . While Myxine (hagfish) is also a jawless cyclostome, Petromyzon is the specific example cited in previous medical entrance exams (RE-AIPMT 2015) for this unique life cycle and larval stage .